Since the 1970s, a growing body of research has suggested that bacteria play an active role in precipitation. These bacteria are capable of catalyzing the formation of ice at relatively warm temperatures utilizing a specific protein family which aids in the binding of water molecules. However, the overall biodiversity, concentration, and relationship of ice nucleation active (ice+) bacteria with air mass trajectories and precipitation chemistry is not well studied. Precipitation events were collected over 15 months in Blacksburg, VA and ice+ bacteria were isolated from these samples. From these samples, 33,134 total isolates were screened for ice nucleation activity (INA) at -8 °C. A total of 593 of these isolated positively confirmed for INA at the same temperature in subsequent tests. The precipitation events had a mean concentration of 384±147 colony forming units per liter. While the majority of confirmed ice+ bacteria belonged to the gammaproteobacteria, a well-studied class of bacteria, including ice+ species of Pseudomonas, Pantoea, and Xanthomonas, two isolates were identified as Lysinibacillus, a Gram-positive member of the Firmicute phylum. These two isolates represent the first confirmed non-gammaproteobacteria with INA. After further characterization, the two isolates of Lysinibacillus did not appear to use a protein to freeze water. Instead, the Lysinibacillus isolates used a secreted, nanometer-sized molecule that is heat, lysozyme, and proteinase resistant. In an attempt to identify the mechanism responsible for this activity, species type strains were tested for INA and UV mutants were generated to knock out the ice+ phenotype. Based on these results, only members of the species L. parviboronicapiens exhibit INA and the genes responsible for the activity may lie within a type-1 polyketide synthase/non-ribosomal peptide synthase gene cluster. This gene cluster is absent from the genomes of all non-ice+ strains of Lysinibacillus, and contains mutations in five of the nine ice nucleation inactive mutants generated from the rain isolated strain. To better understand the phylogenetic relationship among ice+ Lysinibacillus, a comprehensive reference guide was compiled to provide the most up-to-date information regarding the genus and each of its species. This reference will be available to other researchers investigating Lysinibacillus species or other closely related genera. / PHD / It is a common misconception that water freezes at 0°C (32°F). In clouds, water may remain liquid until -37 to -40°C (-35 to -40°F). At temperatures warmer than this, water molecules must collect around small particles that can help form ice, called ice nuclei. Numerous ice nuclei have been identified, ranging from dirt and dust, to volcanic ash, and even to pollen, fungi, and bacteria. One of these bacteria, Pseudomonas syringae, was identified as an ice nucleus in the 1970’s when it was discovered that it was increasing susceptibility of corn to frost damage. Since then, other Pseudomonas species as well as other bacteria within the same family of bacteria have been shown to have the ability to freeze water at relatively warm temperatures utilizing a specialized protein. Despite numerous studies on how these bacteria can exist in the atmosphere and how they can freeze water, the extent of this freezing ability, the concentration of bacteria in precipitation, and how cloud chemistry affects these bacteria has not been widely studied. In this study, precipitation was collected over the course of 15 months and the bacteria found within the collected precipitation were checked to see if they could act as ice nuclei. We found many of the previously described bacterial ice nuclei in the precipitation samples, but also identified a previously unidentified bacterium capable of freezing water. This bacterium, Lysinibacillus parviboronicapiens, does not use the same method of freezing as the other described bacterial ice nuclei. As such, we set out to determine the method it uses. We have determined that this bacterium utilizes a heat-stable, nanometer-sized particle that is not a protein. To better understand this molecule, representative strains of each species of this genus of bacteria were tested for their ability to freeze water, however, only this species has the ability. To further identify the molecule, UV radiation was used to disrupt the bacteria’s ability to freeze water, and the genes responsible were identified. Based on these results, we have tentatively identified the responsible genes as part of a polyketide synthase gene cluster. This gene cluster is responsible for producing small molecules that provide some survival advantage for the bacteria, in our case, possibly the ability to freeze water. As a final step, and to help serve other researchers, a comprehensive analysis of the entire Lysinibacillus genus has been performed and a reference guide has been generated to help describe and distinguish individual species.

This dissertation focuses on identifying and analyzing the mechanism of solid nucleation in liquid thin films. In doing so, we identify and describe a previously unrecognized mechanism of nucleation in condensed systems referred to as transmorphic nucleation. This cluster-shape-change-based mechanism is revealed as a general heterogeneous nucleation mechanism applicable to discontinuous phase transformations occurring in continuous or pre-patterned thin films, as well as in numerous materials systems that possess morphologically and chemically non-trivial heterogeneous-nucleation-catalyzing interfaces (e.g., polycrystalline materials, embedded nano-crystals, and materials with structured interfaces). Identifying, deciphering, and modeling the nature and details associated with how a new phase can nucleate in thin-film materials can be both scientifically meaningful for understanding discontinuous phase transformations in general, and technologically important for engineering various thin-film-based and nano-material-based applications and devices in particular. Classical nucleation theory (CNT) has long been established and regarded as the most practicable treatment that captures the thermodynamic and kinetic essence of the nucleation phenomenon in condensed systems in the simplest and most effective manner. Through a close examination of the theory, we identify and propose morphological equilibrium hypothesis (MEH) as an essential element of CNT. Our shape-transition-based model for transmorphic nucleation in thin films presented in this thesis illustrates that this hypothesis can be violated. As such, the CNT formulation is lacking in capturing the occurrence of the MEH-deviating shape evolution of the clusters, as for instance encountered during the process of transmorphic nucleation. In this dissertation, we conceptually, theoretically, and numerically examine and analyze the kinetic pathway through which nucleation of solids takes place in encapsulated liquid thin films. This example was selected for investigation because it is a particularly simple system, which in turn permits one to make clear, definitive, and general conclusions. A new nucleation mechanism of transmorphic nucleation is discovered in the process. This mechanism is defined generally as the nucleation mechanism through which supercritical clusters are generated from subcritical clusters during an irreversible and morphological-equilibrium-deviating shape evolution initiated when the fluctuating embryos encounter a local growth-inducing element in the catalyzing interface. Both thermodynamic and kinetic analyses in accordance with our transmorphic nucleation mechanism are carried out using a novel adaptation of established theoretical formulations and numerical modeling methods. The kinetic pathway of transmorphic nucleation is described, and transmorphic nucleation temperature window is thermodynamically identified. The kinetic aspect of transmorphic nucleation in thin films is uniquely captured by keeping track of two coupled population distribution profiles of equilibrium-morphology-adhering cluster shapes. Overall, the thesis starts with critical and deconstructive examination of CNT. It builds on our theory of phase initiation and evolution in condensed systems, i.e., Gibbs-Thomson variation (GTV) and Gibbs-Thomson function (GTF), and our interpretation of CNT to investigate steady-state and transient transmorphic nucleation in thin films. The thesis also examines and analyzes all other modes of shape-transition-affected nucleation in thin films outside the transmorphic nucleation domain to provide the comprehensive description of the entire map of nucleation mechanisms in thin-film systems. As far as the implications of the current work on the classical theory of nucleation is concerned, we illustrate how the phenomenon of transmorphic nucleation which violates MEH that forms the basis of CNT, reveals this previously unrecognized limitation of the current formulation of the classical theory of nucleation. The results presented in this dissertation further show that the GTV-based approach, which we identify as the foundation upon which CNT is formulated, can address the MEH-violating shape evolution of subcritical to supercritical clusters. Moreover, the aforementioned reformulation of cluster evolution in this dissertation can be of value for understanding and manipulating phase initiation and evolution involving all of the three Gibbs-Thomson phenomena (i.e., nucleation, coexistence, and free growth) in small, controlled materials systems for optimizing various confined and interface-rich materials that are increasingly becoming technologically important.

Yes / Mandelic acids are prototypic chiral molecules where the sensitivity of crystallized forms (enantiopure/racemic compound/polymorphs) to both conditions and substituents provides a new insight into the factors that may allow chiral separation by crystallization. The determination of a significant number of single crystal structures allows the analysis of 13 enantiopure and 30 racemic crystal structures of 21 (F/Cl/Br/CH3/CH3O) substituted mandelic acid derivatives. There are some common phenyl packing motifs between some groups of racemic and enantiopure structures, although they show very different hydrogen-bonding motifs. The computed crystal energy landscape of 3-chloromandelic acid, which has at least two enantiopure and three racemic crystal polymorphs, reveals that there are many more possible structures, some of which are predicted to be thermodynamically more favorable as well as slightly denser than the known forms. Simulations of mandelic acid dimers in isolation, water, and toluene do not differentiate between racemic and enantiopure dimers and also suggest that the phenyl ring interactions play a major role in the crystallization mechanism. The observed crystallization behavior of mandelic acids does not correspond to any simple “crystal engineering rules” as there is a range of thermodynamically feasible structures with no distinction between the enantiopure and racemic forms. Nucleation and crystallization appear to be determined by the kinetics of crystal growth with a statistical bias, but the diversity of the mandelic acid crystallization behavior demonstrates that the factors that influence the kinetics of crystal nucleation and growth are not yet adequately understood. / EPSRC, Max Planck Society for the Advancement of Sciences, UCL-MPS Impact Ph.D. Fellowship, EU COST Action

Yes / Understanding the effect of physical parameters (e.g., temperature) on crystallisation dynamics is of paramount importance for the synthesis of nanocrystals of well‐defined sizes and geometries. However, imaging nucleation and growth is an experimental challenge owing to the resolution required and the kinetics involved. Here, by using an aberration‐corrected transmission electron microscope, we report the fabrication of precious metal nanocrystals from nuclei and the identification of the dynamics of their nucleation at three different temperatures (20, 50, and 100 °C). A fast, and apparently linear, acceleration of the growth rate is observed against increasing temperature (78.8, 117.7, and 176.5 pm min−1, respectively). This work appears to be the first direct observation of the effect of temperature on the nucleation and growth of metal nanocrystals. / The Royal Society. Grant Number: UF150295 Leverhulme Trust. Grant Number: ECF-2013-414 The Academy of Medical Sciences. Grant Number: SBF003\1170

Yes / This study focuses on numerical modeling of condensation of water vapor in a Laval nozzle, using the liquid drop nucleation theory. Influence of nozzle geometry, pressure, and temperature on the average drop size is reported. A computer program written in MATLAB was used used to calculate the nucleation and condensation of water vapor in the nozzle. The simulation results are validated with the available experimental data in the literature for steam condensation. The model reveals that the average drop size is reduced by increasing the divergent angle of the nozzle. The results also confirm that increasing the inlet pressure has a direct effect on the average drop size while temperature rise has an inverse effect on the drop size.

No description available.

Usant de certaines conditions, la croissance épitaxiale de GaN sur un large panel de substrats donne lieu à une assemblée de nanofils. Cette géométrie filaire peut permettre la croissance d'hétérostructures libres de tous défauts cristallins étendus, ce qui les rendent attractives pour créer des dispositifs de hautes performances. En premier lieu, mon travail de thèse a visé à clarifier le mécanisme de nucléation auto-organisé des nanofils de GaN sur substrat de silicium. Dans ce but, une étude approfondie de la couche tampon d'AlN, déposée préalablement à la nucléation des nanofils, a été réalisée, mettant en évidence une inattendue forte réactivité de l'Al avec le substrat. La nécessité de la polarité azote pour la croissance des nanofils de GaN a été mise en lumière, bien que des nanofils contenant dans leur cœur un domaine de polarité Ga ont également été observés. Dans ces nanofils, une paroi d'inversion de domaine est présente et a été démontrée être optiquement active, exhibant une photoluminescence à 3.45 eV. Ensuite des hétérostuctures filaires GaN/AlN ont été synthétisée pour des caractérisations structurales et optiques. Il a été montré que le mode de croissance de l'hétérostructure peut être changé en fonction du diamètre du nanofil. En dernier lieu, en prenant avantage de la géométrie cylindrique des nanofils, des mesures de diffusion de porteurs de charge ont été réalisées dans des nanofils de GaN et d'AlN. / Using specific conditions, GaN can be epitaxially grown on a large variety of substrates as a nanowire (NW) array. This geometry allows the subsequent growth of wire-like heterostructures likely free of extended defects, which makes them promising for increasing device controllability and performance. First, my PhD work has been devoted to the understanding of self-organized nucleation of GaN NWs on silicon substrates. For this purpose, a deep characterization of the growth mechanism of the AlN buffer deposited prior to NW nucleation has been done, emphasizing an unexpected large reactivity of Al with the substrate. The requirement of the N polarity to nucleate GaN NWs has been evidenced, although the possible existence of NWs hosting a Ga polar core has been observed as well. In these NWs, an inversion domain boundary is present and has been demonstrated to be optically active, having a photoluminescence signature at 3.45 eV. Next, GaN/AlN wire heterostructures have been grown for structural and optical characterization. It has been shown that by changing the wire diameter, different growth mode for the heterostructure could be reached.At last, thanks to the cylindrical geometry of NWs, the measurement of diffusion length for charge carriers in GaN and AlN NWs have been performed.

No description available.

Many experiments demonstrate that the effects of hydrogen solutes decrease macroscopic fracture stresses and strains in ductile materials. Hydrogen-related failures have occurred in nearly all industries involving hydrogen-producing environments. The financial losses incurred from those failures reaches millions if not billions of dollars annually. With the ever-urgent needs for alternative energy sources, there is a strong push for a hydrogen economy from government and private sectors. Safe storage and transportation of hydrogen increases the momentum for studying hydrogen-related failures, especially in ductile materials. To quantify ductile material damage with the effects of hydrogen embrittlement, it is necessary to add hydrogen effects into the void nucleation, void growth, and void coalescence equations. In this research, hydrogen-enhanced void nucleation is our focus, with hydrogen-enhanced void growth and void coalescence t be studied in the future. Molecular Dynamic (MD) and Monte Carlo (MC) simulations with Embedded Atom Method (EAM) potentials were performed to study how hydrogen affects dislocation nucleation, dislocation structure formation and nanovoid nucleation at nickel grain boundaries. The results were inserted into the continuum void nucleation model by Horstemeyer and Gokhale, and the relationships between stress triaxiality-driven void nucleation, grain boundary hydrogen concentrations and local grain geometries were extracted. MD and MC simulations with EAM potentials were also performed to study how hydrogen interstitials affect the dislocation nucleation, dislocation structure formation and subsequent anovoid nucleation of single crystal nickel in different hydrogen-charging conditions. Evolutions of dislocation structures of nickel single crystal with different hydrogen concentrations were compared. The effects of nanovoid nucleation stress and strain at different hydrogen concentrations were quantified. The results were also inserted into the Horstemeyer and Gokhale model and the relationship between stress triaxiality-driven void nucleation and hydrogen concentration caused by stress gradient, which showed similar trends as the grain boundary studies. From nanoscale studies and existing experimental observations, a continuum void nucleation model with hydrogen effects was proposed and used in a continuum damage model based upon Bammann and coworkers. The damage model was implemented into user material code in FEA code ABAQUS. Finite element analyses were performed and the results were compared to the experimental data by Kwon and Asaro.

No description available.